Electro-optical addressing structures are employed in a variety of applications including video cameras, data storage devices, and flat panel liquid crystal displays. Such addressing structures typically include very large numbers of address locations arranged in an array. For example, a flat panel liquid crystal display configured in accordance with a high-definition television format would typically include at least two million address locations. The address locations would correspond to display elements or pixels that are arranged in about 1,000 lines with about 2,000 pixels each.
One object in the design of liquid crystal displays is to minimize cross talk. Adjacent pixels in such a display are closely spaced and have incidental capacitive couplings resulting from these small spacings. Such coupling between adjacent pixels will be referred to as "side-to-side" coupling. In addition, during operation of electro-optical addressing structures, the data drive signals for all the pixels in a column are typically carried on a common conductor adjacent to the pixels. The electrical properties of the electro-optical addressing structures result in capacitive coupling among all the pixels in the column or row. Such coupling among all pixels in a column or row will be referred to as "front-to-back" coupling. A third type of crosstalk, known as "horizontal crosstalk," occurs in a plasma addressed display and is caused by one of the plasma forming electrodes being maintained at a floating potential. Horizontal crosstalk can be eliminated by connecting the floating electrode to a reference electrode, as described in my concurrently filed, copending patent application for "Electrode Shunt in Plasma Channel" of Ilcisin, which is assigned to the assignee of the present invention. These three types of crosstalk cause the data drive signal directed to a particular pixel to be carried to other pixels as incidental data signals or cross talk. For a display system, the cross talk is image dependent, i.e., it depends on the data drive signals present on the conductors and changes the voltage across a specific pixel. Cross talk effects include an unpredictable gray scale that limits the number of achievable gray levels below the number necessary for acceptable video performance. A gray level is sensitive to small variations in the root means square average voltage ("RMS") across a display element, and the cross talk changes that voltage. It will be appreciated that "gray scale" in this context refers to the range of available light output levels in either monochrome or color display systems.
One type of addressing structure is a plasma addressed liquid crystal ("PALC") display described in U.S. Pat. No. 4,896,149 of Buzak et al. for "Addressing Structure Using Ionizable Gaseous Medium," which is assigned to the assignee of the present application. A PALC display places a voltage related to a desired gray level onto a data electrode on one side of a pixel, while ionized gas in a plasma channel provides an electrical path to a reference electrode on the opposite side of the pixel. The voltage remains stored across the pixel when the plasma is extinguished, and the path to ground is eliminated. A PALC display tends to be sensitive to side-to-side cross talk because of the relatively large distance from the data electrode to the reference electrode within the plasma channel. A PALC display also requires the use of higher drive voltages than some other types of display, because the physical components of the display divide the voltage placed on the data electrode, resulting in only a portion of the applied voltage being stored across the liquid crystal to probe the desired gray scale. Higher drive voltages necessitate data drivers having a greater dynamic range and increases the voltage gradient between adjacent pixels in different optical states, thereby exacerbating crosstalk.
Most methods of reducing cross talk in liquid crystal displays entail modifying the data signals in a manner that reduces the incidental signals. One such method is described in U.S. patent application Ser. No. 07/958,631 of Ilcisin et al. for "Adaptive Drive Waveform for Reducing Cross Talk Effects in Electro-Optical Addressing Structures," (hereafter referred to as the "Adaptive Drive Waveform") which is assigned to the assignee of the present invention. Another method for reducing cross talk by varying the addressing signals is described in U.S. Pat. No. 4,845,482 of Howard et al. for "Method for Eliminating Crosstalk in a Thin Film Transistor/Liquid Crystal Display," All such addressing methods also strive to eliminate any direct current bias across the liquid crystal. Direct current biases electrochemically degrade the liquid crystal, resulting in poor image quality.
Cross talk may also be reduced, as described in U.S. patent application Ser. No. 07/854,145 of Buzak for "Reducing Cross Talk Effects in Electro-optical Addressing Structures," which is assigned to the assignee of the present application. The Buzak application describes using a two-phase addressing method in conjunction with a liquid crystal material that is insensitive to voltages at the high frequency of the two-phase signals. Such frequency-sensitive liquid crystals are not, however, suitable for all applications.
Another object in the design of liquid crystal displays is to minimize the voltage requirements. A known method of reducing voltage requirements is to use a liquid crystal material having low threshold voltage, that is, a liquid crystal that begins to change its optical state at a low voltage. A liquid crystal having a low threshold also typically has a low saturation voltage, i.e., a low voltage at which the crystal is considered to be in an "ON" state. The threshold voltage is related to the dielectric properties of the liquid crystal molecules. Most liquid crystal molecules exhibit dielectric anisotropy, that is, the permissivity or dielectric constant (".epsilon.") is different along different molecular axes. The time average direction of the long molecular axis is known as the "director." The dielectric anisotropy (".DELTA..epsilon.") is defined as the difference between the dielectric constant parallel to the director (".epsilon..sub..parallel. ") and the dielectric constant perpendicular (".epsilon..sub.195 ") to the director. Modern liquid crystal display panels typically use liquid crystals having positive dielectric anisotropy, that is, the stronger dipole of the liquid crystal molecules are aligned with the long axis of the molecule.
In the absence of an electric field, liquid crystal molecules typically maintain an orientation determined by the liquid crystal itself, the cell geometry, and the alignment layers. When a voltage applied across the liquid crystal cell produces an electric field exceeding the threshold voltage, the orientation of the liquid crystal molecules and, therefore, the optical transmission properties of the liquid crystal, begin to change. The threshold voltage of a liquid crystal cell varies inversely with the square root of the dielectric anisotropy, .DELTA..epsilon., of the liquid crystal material. To reduce the voltage requirements of a display, display designers typically select a liquid crystal material having a large .DELTA..epsilon. to reduce the threshold voltage.